Just looking by as a wanderer from the boat section via the link there.
Capacitors and Condensers are the same thing. It just depends on when and where you did your learning about electrical devices. They have the property of blocking DC and passing AC, and of storing a charge, and come in many varieties. In this case, the electrolytics act to smooth out voltage supply variations due to instant loading at the power input of the ESC. I suspect that they are paralleled to give a 1000uF capacitor equivalent. The actual tolerances of value on electrolytics are quite wide, +/- 50% is common. It is good practice to run them on about half their rated voltage to allow for any high voltage spikes coming down the supply, so 4-6S sounds about right.
If the guess at it being originally a 90A ESC with water cooling is right, it will need either a generous finned heatsink for air cooling, or generously derating. As to voltage, there might well be other components limiting what can be used.